1. Field of the Invention
The invention relates generally to electronic devices for air navigation and more particularly to radio guidance systems of the type providing air-derived angular information from reception of signals from a ground beacon system of the scanning beam type.
2. Description of the Prior Art
In the prior art, scanning beam radio navigation systems of the type to which the invention applies are known. In addition to the transmission of a single-frequency collimated beam which sweeps the sector of interest, there is either also transmitted a reference signal offset in frequency from that of the beam frequency, as described in copending U.S. patent application, C. W. Earp 156, Ser. No. 662,342, filed Mar. 1, 1976, now U.S. Pat. No. 4,053,892, or there is generated at the receiver a reference signal offset in frequency from that of the beam frequency described hereinafter with reference to FIG. 1 of the accompanying drawings.
The provision of such a reference signal permits a beat between the two signals (beam and reference) to be detected in the receiver to produce a signal of average frequency which is equal to the offset frequency f.sub.o. The instantaneous phase of the beat signal at any point in time depends on the phase relationship between the reference signal and the beam signal, and provides a detected signal which is of a coherent nature.